1. Field of the Invention
The present invention relates to a light diffuser most suitable for use as a plane lighting device. More particularly, the present invention relates to an edge lighting type light diffuser useful as a plane lighting device.
2. Description of the Related Art
Conventionally, large-sized plane lighting units are used as signboard and other various large display devices, and small-sized plane lighting units are used as display devices for various types of machinery and apparatus, and for liquid crystal watches. In signboards and large-sized display devices, a plane to be illuminated is generally illuminated by a plurality of line light sources, for example, fluorescent lighting, or point light sources, e.g., incandescent lamps, which are located behind a light diffusing plate. However, to obtain a plane illumination having a uniform brightness with such plane lighting units, the light diffusing plate and the light source must be spaced quite far apart, or a plurality of light sources must be located in close contact with each other. This requirement necessarily results in an enlargement in the size of the plane lighting unit or an increase in the number of light sources, with the results that the plane lighting unit not only becomes costly but there is also a significant increase in the power consumption.
In an attempt to overcome these deficiencies, some plane lighting units use a single transparent light conducting plate through which light is conducted from one side, thereby obtaining plane illumination. Where such a transparent light conducting plate is used, if the incident light rays from the light source are parallel, it is possible to obtain plane illumination having a substantially uniform brightness. However, since the light from the light source is practically diffused in a radial manner, the quantity of light emitted from the transparent light conducting plate is very bright in the vicinity of the light source and relatively dark at a location remote from the light source. Thus, in the prior art, it has been difficult to obtain uniform plane illumination.